If birds in a truck fly, does the truck get lighter?

It’s an urban myth that had US TV show Mythbusters weighing a truck full of pigeons on a scale and getting them to fly. Now it seems there’s some truth to the idea that a truck driver carrying a cargo of birds can lighten the load by making the birds fly.

But the effect is very short-lived – and worse, immediately afterwards, the avian cargo might actually be up to twice as heavy as the combined weight of the birds.

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So says David Lentink, a mechanical engineer at Stanford University whose team has designed a system for measuring the aerodynamic forces generated by an untethered bird in flight. The system could also be used to test the performance of biologically inspired drones.

Bathroom scales won’t do

Weighing a bird on the wing isn’t easy, says Lentink. As it flaps its wings, it pushes on the air both above and below, meaning its effective weight changes. As an extra complication, the way it moves the air changes several times a second. “Your bathroom scales take a few seconds to determine your weight,” he says. “We needed a system that was super-fast in comparison.”

What they came up with is essentially a box with weighing scales on all of its interior walls that precisely measure fluctuations in air pressure 100 times each second.

By filming Pacific parrotlets (Forpus coelestis) taking off and landing inside the box, and measuring the forces the birds generated as they did so, Lentink’s team confirmed that generalist birds like this generate almost no vertical force when they flap their wings upwards – meaning they really are effectively weightless.

On the downstroke, though, the birds push on the surrounding air so forcefully that they generate a vertical force of up to twice their body weight.

“So if the birds in the cargo all flap at the same time, then the lorry would have to support twice the weight of the birds during the downstroke – and none of their weight during the upstroke,” says Lentink. In reality, the birds will tend to flap at different times, meaning the cargo will, on average, maintain a stable weight.

Change of direction

The result isn’t entirely surprising. “When you look at the downstroke muscle it’s much bigger than the upstroke muscle so it makes sense that the downstroke is doing more,” says Lentink. “But finding out birds do almost all their load lifting on the downstroke and almost none on the upstroke is pretty cool.”

More importantly the new measurement system could be invaluable for the drone industry.

“We’re really at the start of something here,” says Lentink. It’s already possible to assess the aerodynamic performance of drones by tethering them to sensors, he says. But the tether will create noise in measurements, particularly when the drone is changing direction, which is precisely when we need to know the most about their aerodynamic performance.

Jim Usherwood at the Royal Veterinary College in Hatfield, UK, says the new system might be particularly useful for studying drones that have flapping or flexible wings, like birds, bats and insects, because those movements change the centre of mass in a way that makes it tricky to work out their aerodynamic performance.

“The nice thing with this method is that it’s totally insensitive to those centre of mass changes,” says Lentink.